The role of protein synthesis during the labile phases of memory: revisiting the skepticism

The Molecular Substrates of Second-Order Conditioned Fear in the Basolateral Amygdala Complex

Rats quickly learn to fear a stimulus (e.g., a light) that signals brief but painful footshock. The consolidation of this first-order conditioned fear requires transcription and translation of specific genes in the basolateral amygdala complex (BLA). Rats also learn to fear the associates of first-order conditioned stimuli, such as a sound paired with the already-conditioned light. The consolidation of this second-order conditioned fear also requires transcriptional processes in the BLA, but unlike consolidation of first-order fear, it is unaffected by BLA infusions of the protein synthesis inhibitor, cycloheximide. Accordingly, this study sought to identify genes/pathways that regulate second-order conditioned fear in male rats. It focused on the involvement of immediate early genes, as these can be transcribed in the presence of protein synthesis inhibitors. In Experiment 1, we used a principal components analysis of PCR data and found that second-order fear conditioning involves region- and time-specific changes in a network of genes including the immediate early genes bdnf and egr1 In Experiments 2-4, we used BLA infusions of antisense oligonucleotides and found that bdnf is required for consolidation of both first- and second-order fears but is not required for reconsolidation of either and egr1 is not required for consolidation of first-order fear but is required for acquisition of second-order fear and reconsolidation of both types of fear. Hence, bdnf and egr1 regulate different aspects of first- and second-order fear conditioning in the BLA. These findings are discussed with respect to novel forms of information processing in the mammalian brain.

Poly ADP-ribosylation regulates Arc expression and promotes adaptive stress-coping

RATIONALE

Rapid adaptation to stressful events is essential for survival and requires acute stress response and stress-coping strategy. However, the molecular mechanisms that govern this coping strategy have yet to be fully discovered.

OBJECTIVES

This study aims to investigate the effects of poly ADP-ribosylation (PARylation) on stress-coping strategies following acute stress and to identify the target genes influenced by Parp1-induced histone PARylation.

METHODS

Mice were subjected to a forced swim test, a well-established acute stress paradigm, to evaluate cortical PARylation and assess the expression of activity-dependent genes. The pharmacological inhibition of Parp1 was conducted using ABT888 (Veliparib) to determine its effects on stress-coping behavior and related molecular changes.

RESULTS

The forced swim test increased cortical PARylation and upregulated the expression of activity-dependent genes. Systemic inhibition of Parp1 with ABT888 led to impaired stress-coping behavior, evidenced by a reduced immobility response during a subsequent forced swim test done 24 hours later. This impairment was associated with decreased chromatin PARylation and histone H4 acetylation at the Arc promoter and reduced Arc expression observed one hour after Parp1 inhibition.

CONCLUSION

Our findings indicate that chromatin PARylation at the Arc promoters regulates histone H4 acetylation and Arc gene expression, and a subsequent impact on successful stress-coping behavior in response to acute stress.

Gut microbiome-gut brain axis-depression: interconnection

OBJECTIVES

The relationship between the gut microbiome and mental health, particularly depression, has gained significant attention. This review explores the connection between microbial metabolites, dysbiosis, and depression. The gut microbiome, comprising diverse microorganisms, maintains physiological balance and influences health through the gut-brain axis, a communication pathway between the gut and the central nervous system.

METHODS

Dysbiosis, an imbalance in the gut microbiome, disrupts this axis and worsens depressive symptoms. Factors like diet, antibiotics, and lifestyle can cause this imbalance, leading to changes in microbial composition, metabolism, and immune responses. This imbalance can induce inflammation, disrupt neurotransmitter regulation, and affect hormonal and epigenetic processes, all linked to depression.

RESULTS

Microbial metabolites, such as short-chain fatty acids and neurotransmitters, are key to gut-brain communication, influencing immune regulation and mood. The altered production of these metabolites is associated with depression. While progress has been made in understanding the gut-brain axis, more research is needed to clarify causative relationships and develop new treatments. The emerging field of psychobiotics and microbiome-targeted therapies shows promise for innovative depression treatments by harnessing the gut microbiome's potential.

CONCLUSIONS

Epigenetic mechanisms, including DNA methylation and histone modifications, are crucial in how the gut microbiota impacts mental health. Understanding these mechanisms offers new prospects for preventing and treating depression through the gut-brain axis.

Memory engram stability and flexibility

Many studies have shown that memories are encoded in sparse neural ensembles distributed across the brain. During the post-encoding period, often during sleep, many of the cells that were active during encoding are reactivated, supporting consolidation of this memory. During memory recall, many of the same cells that were active during encoding and reactivated during consolidation are reactivated during recall. These ensembles of cells have been referred to as the memory engram cells, stably representing a specific memory. However, recent studies question the rigidity of the "stable memory engram." Here we review the past literature of how episodic-like memories are encoded, consolidated, and recalled. We also highlight more recent studies (as well as some older literature) that suggest that these stable memories and their representations are much more dynamic and flexible than previously thought. We highlight some of these processes, including memory updating, reconsolidation, forgetting, schema learning, memory-linking, and representational drift.

Structural brain preservation: a potential bridge to future medical technologies

When faced with the prospect of death, some people would prefer a form of long-term preservation that may allow them to be restored to healthy life in the future, if technology ever develops to the point that this is feasible and humane. Some believe that we may have the capacity to perform this type of experimental preservation today-although it has never been proven-using contemporary methods to preserve the structure of the brain. The idea is that the morphomolecular organization of the brain encodes the information required for psychological properties such as personality and long-term memories. If these structures in the brain can be maintained intact over time, this could theoretically provide a bridge to access restorative technologies in the future. To consider this hypothesis, we first describe possible metrics that can be used to assess structural brain preservation quality. We next explore several possible methods to preserve structural information in the brain, including the traditional cryonics method of cryopreservation, as well as aldehyde-stabilized cryopreservation and fluid preservation. We focus in-depth on fluid preservation, which relies on aldehyde fixation to induce chemical gel formation in a wide set of biomolecules and appears to be a cost-effective method. We describe two theoretical recovery technologies, alongside several of the ethical and legal complexities of brain preservation, all of which will require a prudent approach. We believe contemporary structural brain preservation methods have a non-negligible chance of allowing successful restoration in the future and that this deserves serious research efforts by the scientific community.

The translational inhibitor and amnestic agent emetine also suppresses ongoing hippocampal neural activity similarly to other blockers of protein synthesis

The consolidation of memory is thought to ultimately depend on the synthesis of new proteins, since translational inhibitors such as anisomycin and cycloheximide adversely affect the permanence of long-term memory. However, when applied directly in brain, these agents also profoundly suppress neural activity to an extent that is directly correlated to the degree of protein synthesis inhibition caused. Given that neural activity itself is likely to help mediate consolidation, this finding is a serious criticism of the strict de novo protein hypothesis of memory. Here, we test the neurophysiological effects of another translational inhibitor, emetine. Unilateral intra-hippocampal infusion of emetine suppressed ongoing local field and multiunit activity at ipsilateral sites as compared to the contralateral hippocampus in a fashion that was positively correlated to the degree of protein synthesis inhibition as confirmed by autoradiography. This suppression of activity was also specific to the circumscribed brain region in which protein synthesis inhibition took place. These experiments provide further evidence that ongoing protein synthesis is necessary and fundamental for neural function and suggest that the disruption of memory observed in behavioral experiments using translational inhibitors may be due, in large part, to neural suppression.

Isoform-specific effects of neuronal inhibition of AMPK catalytic subunit on LTD impairments in a mouse model of Alzheimer's disease

Synaptic dysfunction is highly correlated with cognitive impairments in Alzheimer's disease (AD), the most common dementia syndrome in the elderly. Long-term potentiation (LTP) and long-term depression (LTD) are two primary forms of synaptic plasticity with opposite direction of synaptic efficiency change. Both LTD and LTD are considered to mediate the cellular process of learning and memory. Substantial studies demonstrate AD-associated deficiency of both LTP and LTD. Meanwhile, the molecular signaling mechanisms underlying impairment of synaptic plasticity, particularly LTD, are poorly understood. By taking advantage of the novel transgenic mouse models recently developed in our lab, here we aimed to investigate the roles of AMP-activated protein kinase (AMPK), a central molecular senor that plays a critical role in maintaining cellular energy homeostasis, in regulation of LTD phenotypes in AD. We found that brain-specific suppression of the AMPKα1 isoform (but not AMPKα2 isoform) was able to alleviate mGluR-LTD deficits in APP/PS1 AD mouse model. Moreover, suppression of either AMPKα isoform failed to alleviate AD-related NMDAR-dependent LTD deficits. Taken together with our recent studies on roles of AMPK signaling in AD pathophysiology, the data indicate isoform-specific roles of AMPK in mediating AD-associated synaptic and cognitive impairments.

Acute physical exercise prevents memory amnesia caused by protein synthesis inhibition in rats' hippocampus

The benefits of physical exercise (PE) on memory consolidation have been well-documented in both healthy and memory-impaired animals. However, the underlying mechanisms through which PE exerts these effects are still unclear. In this study, we aimed to investigate the role of hippocampal protein synthesis in memory modulation by acute PE in rats. After novel object recognition (NOR) training, rats were subjected to a 30-min moderate-intensity acute PE on the treadmill, while control animals did not undergo any procedures. Using anisomycin (ANI) and rapamycin (RAPA), compounds that inhibit protein synthesis through different mechanisms, we manipulated protein synthesis in the CA1 region of the hippocampus to examine its contribution to memory consolidation. Memory was assessed on days 1, 7, and 14 post-training. Our results showed that inhibiting protein synthesis by ANI or RAPA impaired NOR memory consolidation in control animals. However, acute PE prevented this impairment without affecting memory persistence. We also evaluated brain-derived neurotrophic factor (BDNF) levels after acute PE at 0.5h, 2h, and 12h afterward and found no differences in levels compared to animals that did not engage in acute PE or were only habituated to the treadmill. Therefore, our findings suggest that acute PE could serve as a non-pharmacological intervention to enhance memory consolidation and prevent memory loss in conditions associated with hippocampal protein synthesis inhibition. This mechanism appears not to depend on BDNF synthesis in the early hours after exercise.

Serotonin Signaling in Hippocampus during Initial Cocaine Abstinence Drives Persistent Drug Seeking

The initiation of abstinence after chronic drug self-administration is stressful. Cocaine-seeking behavior on the first day of the absence of the expected drug (Extinction Day 1, ED1) is reduced by blocking 5-HT signaling in dorsal hippocampal cornu ammonis 1 (CA1) in both male and female rats. We hypothesized that the experience of ED1 can substantially influence later relapse behavior and that dorsal raphe (DR) serotonin (5-HT) input to CA1 may be involved. We inhibited 5-HT1A/1B receptors (WAY-100635 plus GR-127935), or DR input (chemogenetics), in CA1 on ED1 to test the role of this pathway on cocaine-seeking persistence 2 weeks later. We also inhibited 5-HT1A or 5-HT1B receptors in CA1 during conditioned place preference (CPP) for cocaine, to examine mechanisms involved in the persistent effects of ED1 manipulations. Inhibition of DR inputs, or 5-HT1A/1B signaling, in CA1 decreased drug seeking on ED1 and decreased cocaine seeking 2 weeks later revealing that 5-HT signaling in CA1 during ED1 contributes to persistent drug seeking during abstinence. In addition, 5-HT1B antagonism alone transiently decreased drug-associated memory performance when given prior to a CPP test, whereas similar antagonism of 5-HT1A alone had no such effect but blocked CPP retrieval on a test 24 h later. These CPP findings are consistent with prior work showing that DR inputs to CA1 augment recall of the drug-associated context and drug seeking via 5-HT1B receptors and prevent consolidation of the updated nondrug context via 5-HT1A receptors. Thus, treatments that modulate 5-HT-dependent memory mechanisms in CA1 during initial abstinence may facilitate later maintenance of abstinence.

Akt activation ameliorates deficits in hippocampal-dependent memory and activity-dependent synaptic protein synthesis in an Alzheimer's disease mouse model

Protein kinase-B (Akt) and the mechanistic target of rapamycin (mTOR) signaling pathways are implicated in Alzheimer's disease (AD) pathology. Akt/mTOR signaling pathways, activated by external inputs, enable new protein synthesis at the synapse and synaptic plasticity. The molecular mechanisms impeding new protein synthesis at the synapse in AD pathogenesis remain elusive. Here, we aimed to understand the molecular mechanisms prior to the manifestation of histopathological hallmarks by characterizing Akt1/mTOR signaling cascades and new protein synthesis in the hippocampus of WT and amyloid precursor protein/presenilin-1 (APP/PS1) male mice. Intriguingly, compared to those in WT mice, we found significant decreases in pAkt1, pGSK3β, pmTOR, pS6 ribosomal protein, and p4E-BP1 levels in both post nuclear supernatant and synaptosomes isolated from the hippocampus of one-month-old (presymptomatic) APP/PS1 mice. In synaptoneurosomes prepared from the hippocampus of presymptomatic APP/PS1 mice, activity-dependent protein synthesis at the synapse was impaired and this deficit was sustained in young adults. In hippocampal neurons from C57BL/6 mice, downregulation of Akt1 precluded synaptic activity-dependent protein synthesis at the dendrites but not in the soma. In three-month-old APP/PS1 mice, Akt activator (SC79) administration restored deficits in memory recall and activity-dependent synaptic protein synthesis. C57BL/6 mice administered with an Akt inhibitor (MK2206) resulted in memory recall deficits compared to those treated with vehicle. We conclude that dysregulation of Akt1/mTOR and its downstream signaling molecules in the hippocampus contribute to memory recall deficits and loss of activity-dependent synaptic protein synthesis. In AD mice, however, Akt activation ameliorates deficits in memory recall and activity-dependent synaptic protein synthesis.

Opto4E-BP, an optogenetic tool for inducible, reversible, and cell type-specific inhibition of translation initiation

The protein kinase mechanistic target of rapamycin complex 1 (mTORC1) is one of the primary triggers for initiating cap-dependent translation. Amongst its functions, mTORC1 phosphorylates eIF4E-binding proteins (4E-BPs), which prevents them from binding to eIF4E and thereby enables translation initiation. mTORC1 signaling is required for multiple forms of protein synthesis-dependent synaptic plasticity and various forms of long-term memory (LTM), including associative threat memory. However, the approaches used thus far to target mTORC1 and its effectors, such as pharmacological inhibitors or genetic knockouts, lack fine spatial and temporal control. The development of a conditional and inducible eIF4E knockdown mouse line partially solved the issue of spatial control, but still lacked optimal temporal control to study memory consolidation. Here, we have designed a novel optogenetic tool (Opto4E-BP) for cell type-specific, light-dependent regulation of eIF4E in the brain. We show that light-activation of Opto4E-BP decreases protein synthesis in HEK cells and primary mouse neurons. In situ , light-activation of Opto4E-BP in excitatory neurons decreased protein synthesis in acute amygdala slices. Finally, light activation of Opto4E-BP in principal excitatory neurons in the lateral amygdala (LA) of mice after training blocked the consolidation of LTM. The development of this novel optogenetic tool to modulate eIF4E-dependent translation with spatiotemporal precision will permit future studies to unravel the complex relationship between protein synthesis and the consolidation of LTM.

Roles of eukaryotic elongation factor 2 kinase (eEF2K) in neuronal plasticity, cognition, and Alzheimer disease

Understanding the molecular signaling mechanisms underlying cognition and neuronal plasticity would provide insights into the pathogenesis of neuronal disorders characterized by cognitive syndromes such as Alzheimer disease (AD). Phosphorylation of the mRNA translational factor eukaryotic elongation factor 2 (eEF2) by its specific kinase eEF2K is critically involved in protein synthesis regulation. In this review, we discussed recent studies on the roles of eEF2K/eEF2 signaling in the context of regulation/dysregulation of cognitive function and synaptic plasticity. We specifically focus on the discussion of recent evidence indicating suppression of eEF2K signaling as a potential novel therapeutic avenue for AD and related dementias (ADRDs).

Impact of Pseudomonas aeruginosa, Bacillus subtilis, Staphylococcus aureus, and Escherichia coli Oral Infusions on Cognitive Memory Decline in Mild Cognitive Impairment

Synaptic plasticity is a result of changes in the neuronal circuits which may result in the formation of protein-dependent (long-term memory (LTM) formation) and protein-independent (short-term memory (STM) formation) memories. This STM formation is based on existing proteins, but LTM formation depends on RNA and protein synthesis within the neuronal cells. This RNA and protein synthesis may depend on stimulus exposure like odour, taste, and other environmental stimuli. The present study is aimed to show the impact of oral bacterial infusions on cognitive memory formation through pre and post-infusive behavioural analysis. The results of the study revealed that oral infusions of Pseudomonas aeruginosa, Bacillus subtilis, Staphylococcus aureus and Escherichia coli result in impaired cognitive learning and memory formation. This impaired cognitive memory formation is shown with the help of two-step (pre and post-infusive) behavioural analysis. Pre-infusive behavioural study shows no decline in cognitive learning and memory formation before oral microbial infusions in a serene habituated environment. After oral microbial infusions, a post-infusive behavioural analysis may reveal a memory decline in the treated group. Comparative two-step behavioural analysis indicates that P. aeruginosa infusions strongly impact cognitive memory decline compared to the other three groups. This cognitive memory decline may happen due to the production of primary/secondary metabolites within the animal gut and their transportation to the CNS  through the blood-brain barrier. The outcome of the present study states that poor oral hygiene plays a significant role in cognitive memory decline concerning mild cognitive impairment (MCI).

Formation characteristics of long-term memory in Bactrocera dorsalis

Studies on insects have contributed significantly to a better understanding of learning and memory, which is a necessary cognitive capability for all animals. Although the formation of memory has been studied in some model insects, more evidence is required to clarify the characteristics of memory formation, especially long-term memory (LTM), which is important for reliably storing information. Here, we explored this question by examining Bactrocera dorsalis, an agricultural pest with excellent learning abilities. Using the classical conditioning paradigm of the olfactory proboscis extension reflex (PER), we found that paired conditioning with multiple trials (>3) spaced with an intertrial interval (≥10 min) resulted in stable memory that lasted for at least 3 d. Furthermore, even a single conditioning trial was sufficient for the formation of a 2-d memory. With the injection of protein inhibitors, protein-synthesis-dependent memory was confirmed to start 4 h after training, and its dependence on translation and transcription differed. Moreover, the results revealed that the dependence of memory on protein translation exhibited a time-window effect (4-6 h). Our findings provide an integrated view of LTM in insects, suggesting common mechanisms in LTM formation that play a key role in the biological basis of memory.

Nociceptor activity induces nonionotropic NMDA receptor signaling to enable spinal reconsolidation and reverse pathological pain

Chronic, pathological pain is a highly debilitating condition that can arise and be maintained through central sensitization. Central sensitization shares mechanistic and phenotypic parallels with memory formation. In a sensory model of memory reconsolidation, plastic changes underlying pain hypersensitivity can be dynamically regulated and reversed following the reactivation of sensitized sensory pathways. However, the mechanisms by which synaptic reactivation induces destabilization of the spinal "pain engram" are unclear. We identified nonionotropic N-methyl-d-aspartate receptor (NI-NMDAR) signaling as necessary and sufficient for the reactive destabilization of dorsal horn long-term potentiation and the reversal of mechanical sensitization associated with central sensitization. NI-NMDAR signaling engaged directly or through the reactivation of sensitized sensory networks was associated with the degradation of excitatory postsynaptic proteins. Our findings identify NI-NMDAR signaling as a putative synaptic mechanism by which engrams are destabilized in reconsolidation and as a potential means of treating underlying causes of chronic pain.

Corticosterone injection into the dorsal and ventral hippocampus impairs fear memory reconsolidation in a time-dependent manner in rats

Reconsolidation is an active process induced following the reactivation of previously consolidated memories. Recent studies suggest brain corticosteroid receptors may participate in the modulation of fear memory reconsolidation. Glucocorticoid receptors (GRs), with 10-fold lower affinity than mineralocorticoid receptors (MRs), are mainly occupied during the peak of the circadian rhythm, and after stress, so they probably have a more critical role than MRs in memory phases during stressful situations. This study investigated the role of dorsal and ventral hippocampal (DH and VH) GRs and MRs on fear memory reconsolidation in rats. Male Wistar rats with surgically implanted bilaterally cannulae at the DH and VH were trained and tested in an inhibitory avoidance task. The animals received bilateral microinjections of vehicle (0.3 µl/side), corticosterone (3 ng/0.3 µl/side), the GRs antagonist RU38486 (3 ng/0.3 µl/side), or the MRs antagonist spironolactone (3 ng/0.3 µl/side) immediately after memory reactivation. Moreover, drugs were injected into VH 90 minutes after memory reactivation. Memory tests were performed 2, 9, 11, and 13 days after memory reactivation. Results indicated that injection of corticosterone into the DH but not VH immediately after memory reactivation significantly impaired fear memory reconsolidation. Moreover, corticosterone injection into VH 90 minutes after memory reactivation impaired fear memory reconsolidation. RU38486 reversed these effects but not spironolactone. These findings indicate that corticosterone injection into the DH and VH via GRs activation impairs the reconsolidation of fear memory in a time-dependent manner.

The associations between DNA methylation and depression: A systematic review and meta-analysis

BACKGROUND

Growing evidence suggests that epigenetic modification is vital in biological processes of depression. Findings from studies exploring the associations between DNA methylation and depression have been inconsistent.

METHODS

A systematical search of EMBASE, PubMed, Web of Science, and PsycINFO databases was conducted to include studies focusing on the associations between DNA methylation and depression (up to November 1st 2021) according to PRISMA guidelines with registration in PROSPERO (CRD42021288664).

RESULTS

A total of 47 studies met inclusion criteria and 31 studies were included in the meta-analysis. This meta-analysis found that genes hypermethylation, including BDNF (OR: 1.15, 95%CI: 1.01-1.32, I² = 90 %), and NR3C1 (OR: 1.43, 95%CI: 1.09-1.87, I² = 88 %) was associated with increased risk of depression. Significant association of SLC6A4 hypermethylation with depression was only found in the subgroup of using original data (OR: 1.09, 95%CI: 1.01-1.19, I² = 52 %). BDNF hypermethylation could increase the risk of depression only in the Asian population (OR: 1.18, 95%CI: 1.01-1.40, I² = 91 %), and significant associations of NR3C1 hypermethylation with depression were found in the group for depressive symptoms (OR: 1.34, 95%CI: 1.08-1.67, I² = 85 %), but not for depressive disorder (OR: 1.89, 95%CI: 0.54-6.55, I² = 94 %).

LIMITATIONS

More studies are needed to explore the factors that might influence the estimates owing to the contextual heterogeneity of the pooling of included studies.

CONCLUSIONS

It is noted that DNA hypermethylation, namely BDNF and NR3C1, is associated with increased risk of depression. The findings in this study could provide some material evidence for preventing and diagnosing of depression.

Development and optimization of an effective method for evaluating habituation learning behavior in larval zebrafish

BACKGROUND

Habituation learning is a simple and conserved behavior in all organisms which could be induced by repeated stimuli. However, no standard and universal methods for training and evaluating the habituation learning behavior in larval zebrafish were available.

NEW METHOD

This study aims to establish effective training and detection protocols for habituation learning behavior in larval zebrafish by using the ViewPoint system. For this purpose, the detection threshold of velocity-a parameter for distinguishing the escape reaction and the spontaneous motion, the detection sensitivity-a parameter for determining the size of the identified object, the number of stimuli, and the age of larvae were optimized to obtain the best performance.

RESULTS

In this study, the optimized parameters were as follows: the detection threshold of velocity at 13, the luminous intensity at 8 %, the detection sensitivity at 32, the number of stimuli at 150, and the age of larvae at 6 dpf. Furthermore, we validated the utility of the established protocol by showing a consistent memory impairment induced by cycloheximide (CHX).

COMPARISON WITH EXISTING METHOD

A similar method was reported previously. However, the equipment used in those assays, including the hardware and software, were neither standard nor universal, which might impede the extensive application of the habituation learning assays. Here, we developed an alternative method for studying the habituation learning behavior in larval zebrafish using the ViewPoint system.

CONCLUSIONS

Our study provided an alternative method for studying the habituation learning behavior in larval zebrafish.

DNA methylation inhibition participates in the anterograde amnesia key mechanism through the suppression of the transcription of genes involved in memory formation in grape snails

The study of the amnesia mechanisms is of both theoretical and practical importance. The mechanisms of anterograde amnesia are the least studied, due to the lack of an experimental model that allows studying this amnesia type molecular and cellular mechanisms. Previously, we found that conditional food aversion memory reconsolidation impairment in snails by NMDA glutamate receptor antagonists led to the amnesia induction, in the late stages of which (>10 days) repeated training did not cause long-term memory formation. In the same animals, long-term memory aversion to a new food type was formed. We characterized this amnesia as specific anterograde amnesia. In the present work we studied the role of epigenetic DNA methylation processes as well as protein and mRNA synthesis in the mechanisms of anterograde amnesia and memory recovery. DNMT methyltransferase inhibitors (iDNMT: zebularine, RG108 (N-Phthalyl-1-tryptophan), and 5-AZA (5-Aza-2'-deoxycytidine)) were used to alter DNA methylation. It was found that in amnesic animals the iDNMT administration before or after shortened repeated training led to the rapid long-term conditional food aversion formation (Ebbinghaus saving effect). This result suggests that amnestic animals retain a latent memory, which is the basis for accelerated memory formation during repeated training. Protein synthesis inhibitors administration (cycloheximide) before or immediately after repeated training or administration of RNA synthesis inhibitor (actinomycin D) after repeated training prevented memory formation under iDNMT action. The earlier protein synthesis inhibitor effect suggests that the proteins required for memory formation are translated from the pre-existing, translationally repressed mRNAs. Thus, we have shown for the first time that the anterograde amnesia key mechanism is DNMT-dependent suppression of the transcription of genes involved in memory mechanisms. Inhibition of DNMT during repeated training reversed these genes expression blockade, opening access to them by transcription factors synthesized during training from the pre-existing mRNAs.

In-Depth Proteomic Analysis of De Novo Proteome in a Mouse Model of Alzheimer's Disease

BACKGROUND

Alzheimer's disease (AD) is the most common dementia syndrome in the elderly characterized by synaptic failure and unique brain pathology. De novo protein synthesis is required for the maintenance of memory and synaptic plasticity. Mounting evidence links impaired neuronal protein synthesis capacity and overall protein synthesis deficits to AD pathogenesis. Meanwhile, identities of AD-associated dysregulation of "newly synthesized proteome" remain elusive.

OBJECTIVE

To investigate de novo proteome alterations in the hippocampus of aged Tg19959 AD model mice.

METHODS

In this study, we combined the bioorthogonal noncanonical amino acid tagging (BONCAT) method with the unbiased large-scale proteomic analysis in acute living brain slices (we name it "BONSPEC") to investigate de novo proteome alterations in the hippocampus of Tg19959 AD model mice. We further applied multiple bioinformatics methods to analyze in-depth the proteomics data.

RESULTS

In total, 1,742 proteins were detected across the 10 samples with the BONSPEC method. After exclusion of those only detected in less than half of the samples in both groups, 1,362 proteins were kept for further analysis. 37 proteins were differentially expressed (based on statistical analysis) between the WT and Tg19959 groups. Among them, 19 proteins were significantly decreased while 18 proteins were significantly increased in the hippocampi of Tg19959 mice compared to WT mice. The results suggest that proteins involved in synaptic function were enriched in de novo proteome of AD mice.

CONCLUSION

Our study could provide insights into the future investigation into the molecular signaling mechanisms underlying AD and related dementias (ADRDs).

Neurotensin receptor 2 is induced in astrocytes and brain endothelial cells in relation to neuroinflammation following pilocarpine-induced seizures in rats

Neurotensin (NT) acts as a primary neurotransmitter and neuromodulator in the CNS and has been involved in a number of CNS pathologies including epilepsy. NT mediates its central and peripheral effects by interacting with the NTSR1, NTSR2, and Sort1/NTSR3 receptor subtypes. To date, little is known about the precise expression of the NT receptors in brain neural cells and their regulation in pathology. In the present work, we studied the cellular distribution of the NTSR2 protein in the rat hippocampus and questioned whether its expression was modulated in conditions of neuroinflammation using a model of temporal lobe epilepsy induced by pilocarpine. This model is characterized by a rapid and intense inflammatory reaction with reactive gliosis in the hippocampus. We show that NTSR2 protein is expressed in hippocampal astrocytes and its expression increases together with astrocyte reactivity following induction of status epilepticus. NTSR2 immunoreactivity is also increased in astrocytes proximal to blood vessels and their end-feet, and in endothelial cells. Proinflammatory factors such as IL1β and LPS induced NTSR2 mRNA and protein in cultured astroglial cells. Antagonizing NTSR2 with SR142948A decreased NTSR2 expression as well as astroglial reactivity. Together, our results suggest that NTSR2 is implicated in astroglial and gliovascular inflammation and that targeting the NTSR2 receptor may open new avenues in the regulation of neuroinflammation in CNS diseases.

Autophagy coupled to translation is required for long-term memory

An increase in protein synthesis following learning is a fundamental and evolutionarily conserved mechanism of long-term memory. To maintain homeostasis, this protein synthesis must be counterbalanced by mechanisms such as protein degradation. Recent studies reported that macroautophagy/autophagy, a major protein degradation mechanism, is required for long-term memory formation. However, how learning regulates autophagy and recruits it into long-term memory formation remains to be established. Here, we show that inhibitory avoidance in rats significantly increases the levels of autophagy and lysosomal degradation proteins, including BECN1/beclin 1, LC3-II, SQSTM1/p62 and LAMP1, as well as autophagic flux in the hippocampus. Moreover, pharmacological inhibition or targeted molecular disruption of the learning-induced autophagy impairs long-term memory, leaving short-term memory intact. The increase in autophagy proteins results from active translation of their mRNA and not from changes in their total mRNA levels. Additionally, the induction of autophagy requires the immediate early gene Arc/Arg3.1. Finally, in contrast to classical regulation of autophagy in other systems, we found that the increase in autophagy upon learning is dispensable for the increase in protein synthesis. We conclude that coupling between learning-induced translation and autophagy, rather than translation per se, is an essential mechanism of long-term memory.Abbreviations: AAV: adeno-associated virus; ARC/ARG3.1: activity regulated cytoskeletal-associated protein; ATG: autophagy related; DG: dentate gyrus; GFP: green fluorescent protein; IA: inhibitory avoidance; LAMP1: lysosomal-associated membrane protein 1; MAP1LC3B/LC3B: microtubule-associated protein 1 light chain 3 beta; ODN: oligodeoxynucleotide; qPCR: quantitative polymerase chain reaction; SBI: SBI0206965; SQSTM1/p62: sequestosome 1; SUnSET: surface sensing of translation; TRAP: translating ribosome affinity purification; ULK1: unc-51 like kinase 1.

Local Translation in Perisynaptic Astrocytic Processes Is Specific and Changes after Fear Conditioning

Local translation is a conserved mechanism conferring cells the ability to quickly respond to local stimuli. In the brain, it has been recently reported in astrocytes, whose fine processes contact blood vessels and synapses. Yet the specificity and regulation of astrocyte local translation remain unknown. We study hippocampal perisynaptic astrocytic processes (PAPs) and show that they contain the machinery for translation. Using a refined immunoprecipitation technique, we characterize the entire pool of ribosome-bound mRNAs in PAPs and compare it with the one expressed in the whole astrocyte. We find that a specific pool of mRNAs is highly polarized at the synaptic interface. These transcripts encode an unexpected molecular repertoire, composed of proteins involved in iron homeostasis, translation, cell cycle, and cytoskeleton. Remarkably, we observe alterations in global RNA distribution and ribosome-bound status of some PAP-enriched transcripts after fear conditioning, indicating the role of astrocytic local translation in memory and learning.

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Local translation occurs in perisynaptic astrocytic processes (PAPs)

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The repertoire of ribosome-bound mRNAs enriched in hippocampal PAPs is specific

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RNA distribution and local translation change in PAPs after fear conditioning

Brain-specific suppression of AMPKα2 isoform impairs cognition and hippocampal LTP by PERK-mediated eIF2α phosphorylation

The AMP-activated protein kinase (AMPK) is a molecular sensor to maintain energy homeostasis. The two isoforms of the AMPK catalytic subunit (AMPKα1 and α2) are both expressed in brains, but their roles in cognition are unknown. We generated conditional knockout mice in which brain AMPKα isoforms are selectively suppressed (AMPKα1/α2 cKO), and determined the isoform-specific effects in mice of either sex on cognition and synaptic plasticity. AMPKα2 cKO but not AMPKα1 cKO displayed impaired cognition and hippocampal late long-term potentiation (L-LTP). Further, AMPKα2 cKO mice exhibited decreased dendritic spine density and abnormal spine morphology in hippocampus. Electron microscope imaging demonstrated reduced postsynaptic density formation and fewer dendritic polyribosomes in hippocampi of AMPKα2 cKO mice. Biochemical studies revealed unexpected findings that repression of AMPKα2 resulted in increased phosphorylation of mRNA translational factor eIF2α and its kinase PERK. Importantly, L-LTP failure and cognitive impairments displayed in AMPKα2 cKO mice were alleviated by suppressing PERK activity pharmacologically or genetically. In summary, we demonstrate here that brain-specific suppression of AMPKα2 isoform impairs cognition and hippocampal LTP by PERK-mediated eIF2α phosphorylation, providing molecular mechanisms linking metabolism, protein synthesis, and cognition.

Metabolic Aspects of Adenosine Functions in the Brain

Adenosine, acting both through G-protein coupled adenosine receptors and intracellularly, plays a complex role in multiple physiological and pathophysiological processes by modulating neuronal plasticity, astrocytic activity, learning and memory, motor function, feeding, control of sleep and aging. Adenosine is involved in stroke, epilepsy and neurodegenerative pathologies. Extracellular concentration of adenosine in the brain is tightly regulated. Adenosine may be generated intracellularly in the central nervous system from degradation of AMP or from the hydrolysis of S-adenosyl homocysteine, and then exit via bi-directional nucleoside transporters, or extracellularly by the metabolism of released nucleotides. Inactivation of extracellular adenosine occurs by transport into neurons or neighboring cells, followed by either phosphorylation to AMP by adenosine kinase or deamination to inosine by adenosine deaminase. Modulation of the nucleoside transporters or of the enzymatic activities involved in the metabolism of adenosine, by affecting the levels of this nucleoside and the activity of adenosine receptors, could have a role in the onset or the development of central nervous system disorders, and can also be target of drugs for their treatment. In this review, we focus on the contribution of 5'-nucleotidases, adenosine kinase, adenosine deaminase, AMP deaminase, AMP-activated protein kinase and nucleoside transporters in epilepsy, cognition, and neurodegenerative diseases with a particular attention on amyotrophic lateral sclerosis and Huntington's disease. We include several examples of the involvement of components of the adenosine metabolism in learning and of the possible use of modulators of enzymes involved in adenosine metabolism or nucleoside transporters in the amelioration of cognition deficits.

Protein synthesis inhibitor administration before a reminder caused recovery from amnesia induced by memory reconsolidation impairment with NMDA glutamate receptor antagonist

Memory recovery in amnestic animals is one of the most poorly studied processes. In this paper, we examine the role of protein synthesis and a reminder in the mechanisms of amnesia and memory recovery in grape snails trained to conditioned food aversion. Amnesia was induced by the impairment of memory reconsolidation using NMDA (N-methyl d-aspartate) glutamate receptor antagonists. In an early stage of amnesia (day 3), injections of protein synthesis inhibitors into animals combined with a reminder by a conditioned stimulus (CS) led to the recovery of aversive reactions to its presentation. Two types of changes in reactions to CS were revealed. In most animals, a persistent recovery of memory retrieval was found that lasted for at least 10 days. In other snails, aversive responses to CS persisted for 24 h. Isolated injections of inhibitors, injections of inhibitors and a reminder by the learning environment (without presenting a CS), usage of a differentiating stimulus instead of a CS, or inhibitor injections after the reminder did not affect the development of amnesia. The administration of protein synthesis inhibitors and a reminder in the late period after amnesia induction (10 days) did not affect its development or caused a short-term memory recovery. We suggest that amnesia is an active process that develops over time. The reminder induces the reactivation of the amnesia process dependent on protein synthesis, while the administration of protein synthesis inhibitors leads to the impairment of amnesia reactivation and recovery of the state formed before amnesia induction (i.e., recovery of conditioned food aversion memory).

On the theory of mental representation block. a novel perspective on learning and behavior

Understanding the mechanisms behind memory, learning, and behavior is crucial to human development and significant research has been done in this area. Classical and operant conditioning and other theories of learning have elucidated different mechanisms of learning and how it modulates behavior. Even with advances in this area, questions remain on how to unlearn faulty ideas or extinguish maladaptive behaviors. In this paper, a novel theory to improve our understanding of this area is proposed. The theory proposes that as a consequence of the brain's energy efficiency evolutionary adaptations, all learning following memory consolidation, reconsolidation, and repeated reinforcements or strengthening over time, results in a phenomenon called mental representation block. The implications of this block on learning and behavior are significant and broad and include cognitive biases, belief in a creator or God, close-mindedness, dogmatism, physician misdiagnosis, racism, homophobia, and transphobia, susceptibility to deception and indoctrination, hate and love, artificial intelligence and creativity.

Suppression of the kinase for elongation factor 2 alleviates mGluR-LTD impairments in a mouse model of Alzheimer's disease

Impaired mRNA translation (protein synthesis) is linked to Alzheimer's disease (AD) pathophysiology. Recent studies revealed the role of increased phosphorylation of eukaryotic elongation factor 2 (eEF2) in AD-associated cognitive deficits. Phosphorylation of eEF2 (at the Thr56 site) by its only known kinase eEF2K leads to inhibition of general protein synthesis. AD is considered as a disease of "synaptic failure" characterized by impairments of synaptic plasticity, including long-term potentiation (LTP) and long-term depression (LTD). Deficiency of metabotropic glutamate receptor 5-dependent LTD (mGluR-LTD) is indicated in cognitive syndromes associated with various neurological disorders, including AD, but the molecular signaling mechanisms underlying the mGluR-LTD dysregulation in AD remain unclear. In this brief communication, we report genetic repression of eEF2K in aged APP/PS1 AD model mice prevented AD-associated hippocampal mGluR-LTD deficits. Using a pharmacological approach, we further observed that impairments of mGluR-LTD in APP/PS1 mice were rescued by treating hippocampal slices with a small molecule eEF2K antagonist NH125. Our findings, taken together, suggest a critical role of abnormal protein synthesis dysregulation at the elongation phase in AD-associated mGluR-LTD failure, thus providing insights into a mechanistic understanding of synaptic impairments in AD and other related dementia syndromes.

Protein expression alteration in hippocampus upon genetic repression of AMPKα isoforms

The AMP-activated protein kinase (AMPK) is a molecular sensor to help maintain cellular energy homeostasis. AMPK is a heterotrimeric complex and its enzymatic catalytic subunit includes two isoforms: α1 and α2. Dysregulation of AMPK signaling is linked to neuronal diseases characterized with cognitive impairments. Emerging evidence also suggest isoform-specific roles of AMPK in the brain. AMPK regulates protein synthesis, which is critical for memory formation and neuronal plasticity. However, the consequence of altering AMPK activity on the translation of specific proteins in the brain is unknown. Here, we use unbiased mass spectrometry-based proteomics approach to analyze protein profile alterations in hippocampus and prefrontal cortex of transgenic mice in which the genes for the two AMPKα isoforms are conditionally deleted. The study revealed identities of proteins whose expression is sensitive to suppression of AMPKα1 and/or α2 isoform. These data may serve as a basis for future in-depth study. Elucidation of the functional relevance of the alteration of specific proteins could provide insights into identification of novel therapeutic targets for neuronal disorders characterized with AMPK signaling dysregulation and impaired cellular energy metabolism.

Two spaced training trials induce associative ERK-dependent long term memory in Neohelice granulata

Memory formation depends upon several parametric training conditions. Among them, trial number and inter-trial interval (ITI) are key factors to induce long-term retention. However, it is still unclear how individual training trials contribute to mechanisms underlying memory formation and stabilization. Contextual conditioning in Neohelice granulata has traditionally elicited associative long-term memory (LTM) after 15 spaced (ITI = 3 min) trials. Here, we show that LTM in crabs can be induced after only two training trials by increasing the ITI to 45 min (2t-LTM) and maintaining the same training duration as in traditional protocols. This newly observed LTM was preserved for at least 96 h, exhibiting protein synthesis dependence during consolidation and reconsolidation as well as context-specificity. Moreover, we demonstrate that 2t-LTM depends on inter-trial and post-training ERK activation showing a faster phosphorylation after the second trial compared to the first one. In summary, we present a new training protocol in crabs through a reduced number of trials showing associative features similar to traditional spaced training. This novel protocol allows for intra-training manipulation and the assessment of individual trial contribution to LTM formation.

The role of intrinsic excitability in the evolution of memory: Significance in memory allocation, consolidation, and updating

Memory is a dynamic process that is continuously regulated by both synaptic and intrinsic neural mechanisms. While numerous studies have shown that synaptic plasticity is important in various types and phases of learning and memory, neuronal intrinsic excitability has received relatively less attention, especially regarding the dynamic nature of memory. In this review, we present evidence demonstrating the importance of intrinsic excitability in memory allocation, consolidation, and updating. We also consider the intricate interaction between intrinsic excitability and synaptic plasticity in shaping memory, supporting both memory stability and flexibility.

Repression of eEF2 kinase improves deficits in novel object recognition memory in aged mice

The normal aging process is commonly associated with mild cognitive deficits including memory decline. Previous studies indicate a role of dysregulated messenger ribonucleic acid translation capacity in cognitive defects associated with aging and aging-related diseases, including hyperphosphorylation of eukaryotic elongation factor 2 (eEF2). Phosphorylation of eEF2 by the kinase eEF2K inhibits its activity, hindering general protein synthesis. Here, we sought to determine whether cognitive deficits in aged mice can be improved by genetically deleting eEF2K (eEF2K KO) and consequently reduction of eEF2 phosphorylation. We found that suppression of eEF2K prevented aging-related deficits in novel object recognition memory. Interestingly, deletion of eEF2K did not alter overall protein synthesis in the hippocampus. Ultrastructural analysis revealed increase size and larger active zone lengths of postsynaptic densities in the hippocampus of aged eEF2K KO mice. Biochemical assays showed hippocampal eIF2α hyperphosphorylation in aged eEF2K KO mice, indicating inhibition of translation initiation. Our findings may provide insight into mechanistic understanding and thus development of novel therapeutic strategies for aging-related cognitive decline.

Memory reacquisition deficit: Chicks fail to relearn pharmacologically disrupted associative response

It is generally assumed that if memory is disrupted by pharmacological inhibitors during its consolidation, it can be later acquired afresh. In our experiments, we trained day-old chicks in a one-trial passive avoidance task and interfered with memory formation using protein synthesis inhibitor anisomycin or NMDA receptor antagonist MK-801. Second training was then given to amnestic animals with either the same conditioning stimulus (retraining) or a new one (novel training). Retraining with the same stimulus failed to produce efficient memory at all the examined between-training and training-to-test intervals, while a new conditioned stimulus was learned successfully. We suggest that this memory reacquisition deficit may result from the failure of associative memory co-allocation mechanisms.

Brain-specific repression of AMPKα1 alleviates pathophysiology in Alzheimer's model mice

AMPK is a key regulator at the molecular level for maintaining energy metabolism homeostasis. Mammalian AMPK is a heterotrimeric complex, and its catalytic α subunit exists in 2 isoforms: AMPKα1 and AMPKα2. Recent studies suggest a role of AMPKα overactivation in Alzheimer's disease-associated (AD-associated) synaptic failure. However, whether AD-associated dementia can be improved by targeting AMPK remains unclear, and roles of AMPKα isoforms in AD pathophysiology are not understood. Here, we showed distinct disruption of hippocampal AMPKα isoform expression patterns in postmortem human AD patients and AD model mice. We further investigated the effects of brain- and isoform-specific AMPKα repression on AD pathophysiology. We found that repression of AMPKα1 alleviated cognitive deficits and synaptic failure displayed in 2 separate lines of AD model mice. In contrast, AMPKα2 suppression did not alter AD pathophysiology. Using unbiased mass spectrometry-based proteomics analysis, we identified distinct patterns of protein expression associated with specific AMPKα isoform suppression in AD model mice. Further, AD-associated hyperphosphorylation of eukaryotic elongation factor 2 (eEF2) was blunted with selective AMPKα1 inhibition. Our findings reveal isoform-specific roles of AMPKα in AD pathophysiology, thus providing insights into potential therapeutic strategies for AD and related dementia syndromes.

Norepinephrine stabilizes translation-dependent, homosynaptic long-term potentiation through mechanisms requiring the cAMP sensor Epac, mTOR and MAPK

Neuromodulators regulate higher-order cognitive processes including learning and memory through modulation of synaptic transmission and plasticity. Norepinephrine is a neuromodulator that is secreted throughout the brain in response to novelty or increased arousal, which alters neural circuits by increasing the modifiability of CNS synapses. Norepinephrine activates metabotropic receptors, initiating complex intracellular signalling cascades that can promote enduring changes in synaptic strength including long-term potentiation (LTP). In particular, activation of beta-adrenergic receptors (β-ARs) by norepinephrine enhances LTP through downstream engagement of signalling cascades which upregulate protein synthesis at synapses. Here, we sought to determine the select signalling pathways recruited by norepinephrine to promote homosynaptic LTP at hippocampal synapses in mice. Application of norepinephrine initiated a long-lasting form of homosynaptic LTP that requires protein synthesis. Norepinephrine-mediated enhancement of LTP was reduced by inhibition of mammalian target of rapamycin and exchange protein directly activated by cAMP (Epac) but not cAMP-dependent protein kinase A, suggesting that the endogenous β-AR ligand norepinephrine may preferentially recruit Epac signalling to promote enduring changes in synaptic strength. These findings advance our understanding of the mechanisms through which norepinephrine regulates synaptic plasticity associated with formation of new memories.

An update on memory formation and retrieval: An engram-centric approach

OBJECTIVE

We explore here that memory loss observed in the early stage of Alzheimer's disease (AD) is a disorder of memory retrieval, instead of a storage impairment. This engram-centric explanation aims to enlarge the conceptual frame of memory as an emergent behavior of the brain and to propose a new treatment strategy for memory retrieval in dementia-AD.

BACKGROUND

The conventional memory hypothesis suggests that memory is stored as multiple traces in hippocampal neurons but recent evidence indicates that there are specialized memory engrams responsible for the storage and the retrieval of different memory types.

UPDATED MEMORY HYPOTHESIS

There are specialized memory engram neurons for each memory type and when information will be stored as a memory arrives in the hippocampus through afferent neurons finds its neuron according to the excitability states of engram neurons. The excitability level in engram neurons seems like a code canalizing the interactions between engrams and information. Therefore, to enhance the excitability of memory engram neurons improves memory loss observed in AD. In addition, we suggest that the hippocampus creates an index for information stored in memory engram cells in specialized regions for different types of memory, instead of storing all information; and different anatomic locations of engram cells and their roles in memory retrieval point out that memory could be an emergent behavior of the brain, and the interaction between serotonin fluctuation and engram neurons could be neural underpinnings of terminal lucidity.

MAJOR CHALLENGES FOR THE MODEL

The major challenge for this engram-centric memory retrieval model is the translation from bench to patient, specifically the delivery of optogenetic tools in patients. Engram neurons can be specifically activated by optogenetic tools, but optogenetics is an invasive technique which requires optic fiber implantation into the brain. In addition, light can overheat the tissue and thus induce damage in tissue. Furthermore, light is a foreign object and its direct implantation into the brain may cause neuroinflammation, the main trigger of neurodegenerative diseases. Therefore, to test the engram hypothesis in human, new tools to allow specific engram activation should be discovered.

The Relationship between DNA Methylation and Antidepressant Medications: A Systematic Review

Major depressive disorder (MDD) is the leading cause of disability worldwide and is associated with high rates of suicide and medical comorbidities. Current antidepressant medications are suboptimal, as most MDD patients fail to achieve complete remission from symptoms. At present, clinicians are unable to predict which antidepressant is most effective for a particular patient, exposing patients to multiple medication trials and side effects. Since MDD’s etiology includes interactions between genes and environment, the epigenome is of interest for predictive utility and treatment monitoring. Epigenetic mechanisms of antidepressant medications are incompletely understood. Differences in epigenetic profiles may impact treatment response. A systematic literature search yielded 24 studies reporting the interaction between antidepressants and eight genes (BDNF, MAOA, SLC6A2, SLC6A4, HTR1A, HTR1B, IL6, IL11) and whole genome methylation. Methylation of certain sites within BDNF, SLC6A4, HTR1A, HTR1B, IL11, and the whole genome was predictive of antidepressant response. Comparing DNA methylation in patients during depressive episodes, during treatment, in remission, and after antidepressant cessation would help clarify the influence of antidepressant medications on DNA methylation. Individuals’ unique methylation profiles may be used clinically for personalization of antidepressant choice in the future.

Brain-penetrant PQR620 mTOR and PQR530 PI3K/mTOR inhibitor reduce huntingtin levels in cell models of HD

One of the pathological hallmarks of Huntington disease (HD) is accumulation of the disease-causing mutant huntingtin (mHTT), which leads to the disruption of a variety of cellular functions, ultimately resulting in cell death. Induction of autophagy, for example by the inhibition of mechanistic target of rapamycin (mTOR) signaling, has been shown to reduce HTT levels and aggregates. While rapalogs like rapamycin allosterically inhibit the mTOR complex 1 (TORC1), ATP-competitive mTOR inhibitors suppress activities of TORC1 and TORC2 and have been shown to be more efficient in inducing autophagy and reducing protein levels and aggregates than rapalogs. The ability to cross the blood-brain barrier of first generation catalytic mTOR inhibitors has so far been limited, and therefore sufficient target coverage in the brain could not be reached. Two novel, brain penetrant compounds - the mTORC1/2 inhibitor PQR620, and the dual pan-phosphoinositide 3-kinase (PI3K) and mTORC1/2 kinase inhibitor PQR530 - were evaluated by assessing their potential to induce autophagy and reducing mHTT levels. For this purpose, expression levels of autophagic markers and well-defined mTOR targets were analyzed in STHdh cells and HEK293T cells and in mouse brains. Both compounds potently inhibited mTOR signaling in cell models as well as in mouse brain. As proof of principle, reduction of aggregates and levels of soluble mHTT were demonstrated upon treatment with both compounds. Originally developed for cancer treatment, these second generation mTORC1/2 and PI3K/mTOR inhibitors show brain penetrance and efficacy in cell models of HD, making them candidate molecules for further investigations in HD.

Memory Decline and Behavioral Inflexibility in Aged Mice Are Correlated With Dysregulation of Protein Synthesis Capacity

Understanding the molecular mechanisms underlying age-associated cognitive impairments will not only contribute to our general knowledge about "aging" biology, but also provide insights for more effective strategies to prevent and improve the quality of life for both normal aging and pathological aging such as Alzheimer's disease (AD). Here we first assessed and compared the performance of cognition and synaptic plasticity in young (3-5-month old) and aged c57BL/6J mice (19-21 months old). Findings from behavioral tests demonstrated that old mice, compared to young mice, displayed impairments in spatial learning/memory, working memory, and behavioral flexibility. Further, synaptic electrophysiology experiments on hippocampal slices revealed that the early form of long-term potentiation (LTP, a synaptic model for memory formation) was inhibited in old mice. At the molecular level, biochemical assays on the hippocampus showed dysregulation of signaling pathways controlling protein synthesis capacity including: up-regulation of AKT-mTORC1-p70S6K signaling, which is associated with translation of terminal oligopyrimidine (TOP) class of mRNAs that encode translational machinery; hyper-phosphorylation of mRNA translational elongation factor 2 (eEF2) and its upstream regulator AMP-activated protein kinase (AMPK), indicating repression of general protein synthesis. Moreover, young and old mice exhibited similar brain levels of translational initiation factor 2α (eIF2α) phosphorylation, which is known to be increased in AD and linked to the disease pathophysiology. Thus, our data provide evidence at the molecular level to highlight the similarity and difference between normal and pathological aging, which may contribute to future studies on diagnostic/prognostic biomarkers for aging-related dementia syndromes.

Genetic removal of eIF2α kinase PERK in mice enables hippocampal L-LTP independent of mTORC1 activity

Characterization of the molecular signaling pathways underlying protein synthesis-dependent forms of synaptic plasticity, such as late long-term potentiation (L-LTP), can provide insights not only into memory expression/maintenance under physiological conditions but also potential mechanisms associated with the pathogenesis of memory disorders. Here, we report in mice that L-LTP failure induced by the mammalian (mechanistic) target of rapamycin complex 1 (mTORC1) inhibitor rapamycin is reversed by brain-specific genetic deletion of PKR-like ER kinase, PERK (PERK KO), a kinase for eukaryotic initiation factor 2α (eIF2α). In contrast, genetic removal of general control non-derepressible-2, GCN2 (GCN2 KO), another eIF2α kinase, or treatment of hippocampal slices with the PERK inhibitor GSK2606414, does not rescue rapamycin-induced L-LTP failure, suggesting mechanisms independent of eIF2α phosphorylation. Moreover, we demonstrate that phosphorylation of eukaryotic elongation factor 2 (eEF2) is significantly decreased in PERK KO mice but unaltered in GCN2 KO mice or slices treated with the PERK inhibitor. Reduction in eEF2 phosphorylation results in increased general protein synthesis, and thus could contribute to the mTORC1-independent L-LTP in PERK KO mice. We further performed experiments on mutant mice with genetic removal of eEF2K (eEF2K KO), the only known kinase for eEF2, and found that L-LTP in eEF2K KO mice is insensitive to rapamycin. These data, for the first time, connect reduction in PERK activity with the regulation of translation elongation in enabling L-LTP independent of mTORC1. Thus, our findings indicate previously unrecognized levels of complexity in the regulation of protein synthesis-dependent synaptic plasticity. Read the Editorial Highlight for this article on page 119. Cover Image for this issue: doi: 10.1111/jnc.14185.

Cocaine conditioned place preference: unexpected suppression of preference due to testing combined with strong conditioning

Conditioned place preference (CPP) is widely used for evaluating the rewarding effects of drugs. Like other memories, CPP is proposed to undergo reconsolidation during which it is unstable and sensitive to pharmacological inhibition. Previous studies have shown that cocaine CPP can be apparently erased by extracellular signal-regulated kinase (ERK) pathway inhibition during cocaine reconditioning (re-exposure to the drug-paired environment in the presence of the drug). Here, we show that blockade of D1 receptors during reconditioning prevented ERK activation and induced a loss of CPP. However, we also unexpectedly observed a CPP disappearance in mice that underwent testing and reconditioning with cocaine alone, specifically in strong conditioning conditions. The loss was due to the intermediate test. CPP was not recovered with reconditioning or priming in the short term, but it spontaneously reappeared after a month. When we challenged the D1 antagonist-mediated erasure, we observed that both a high dose of cocaine and a first CPP test were required for this effect. Our results also suggest a balance between D1-dependent ERK pathway activation and an A2a-dependent mechanism in D2 striatal neurons in controlling CPP expression. Our data reveal that, paradoxically, a simple CPP test can induce a complete (but transient) loss of place preference following strong but not weak cocaine conditioning. This study emphasizes the complex nature of CPP memory and the importance of multiple parameters that must be taken into consideration when investigating reconsolidation.

Protein synthesis inhibitors induce both memory impairment and its recovery

The involvement of protein synthesis in the mechanisms of conditioned food aversion memory impairment and recovery in grape snails was studied. It was found that protein synthesis inhibitor (cycloheximide) injections before a reminder by the conditioned stimulus (CS) caused amnesia development. Three days after amnesia induction, injections of cycloheximide or another protein synthesis inhibitor, anisomycin, combined with a reminder by four CSs resulted in memory retrieval, which was saved for 24 h. Cycloheximide injections and the administration of one CS as a reminder to an amnestic animals caused the memory expression only in response to this CS, while it was absent the next day. The isolated administration of a reminder or inhibitor injections without a reminder was not effective. It is suggested that amnesia is an active process and that one of its mechanisms may be a protein-dependent amnesia reactivation caused by a reminder. The administration of protein synthesis inhibitors led to impairment of amnesia reactivation and to recovery of the state formed before amnesia induction and thus to the recovery of conditioned food aversion memory.

Early memory consolidation window enables drug induced state-dependent memory

It is well established that newly acquired information is stabilized over time by processes underlying memory consolidation, these events can be impaired by many drug treatments administered shortly after learning. The consolidation hypothesis has been challenged by a memory integration hypothesis, which suggests that the processes underlying new memories are vulnerable to incorporation of the neurobiological alterations induced by amnesic drugs generating a state-dependent memory. The present experiments investigated the effects of amnesic drugs infused into the insular cortex of male Wistar rats on memory for object recognition training. The findings provide evidence that infusions of several amnesic agents including a protein synthesis inhibitor, an RNA synthesis inhibitor, or an NMDA receptor antagonist administered both after a specific period of time and before retrieval induce state-dependent recognition memory. Additionally, when amnesic drugs were infused outside the early consolidation window, there was amnesia, but the amnesia was not state-dependent. Data suggest that amnesic agents can induce state-dependent memory when administered during the early consolidation window and only if the duration of the drug effect is long enough to become integrated to the memory trace. In consequence, there are boundary conditions in order to induce state-dependent memory.

Mechanisms of disordered neurodegenerative function: concepts and facts about the different roles of the protein kinase RNA-like endoplasmic reticulum kinase (PERK)

Neurodegenerative diseases, such as Alzheimer's disease, Huntington's disease, Parkinson's disease, prion disease, and amyotrophic lateral sclerosis, are a dissimilar group of disorders that share a hallmark feature of accumulation of abnormal intraneuronal or extraneuronal misfolded/unfolded protein and are classified as protein misfolding disorders. Cellular and endoplasmic reticulum (ER) stress activates multiple signaling cascades of the unfolded protein response (UPR). Consequently, translational and transcriptional alterations in target gene expression occur in response directed toward restoring the ER capacity of proteostasis and reestablishing the cellular homeostasis. Evidences from in vitro and in vivo disease models indicate that disruption of ER homeostasis causes abnormal protein aggregation that leads to synaptic and neuronal dysfunction. However, the exact mechanism by which it contributes to disease progression and pathophysiological changes remains vague. Downstream signaling pathways of UPR are fully integrated, yet with diverse unexpected outcomes in different disease models. Three well-identified ER stress sensors have been implicated in UPR, namely, inositol requiring enzyme 1, protein kinase RNA-activated-like ER kinase (PERK), and activating transcription factor 6. Although it cannot be denied that each of the involved stress sensor initiates a distinct downstream signaling pathway, it becomes increasingly clear that shared pathways are crucial in determining whether or not the UPR will guide the cells toward adaptive prosurvival or proapoptotic responses. We review a body of work on the mechanism of neurodegenerative diseases based on oxidative stress and cell death pathways with emphasis on the role of PERK.

Memory corticalization triggered by REM sleep: mechanisms of cellular and systems consolidation

Once viewed as a passive physiological state, sleep is a heterogeneous and complex sequence of brain states with essential effects on synaptic plasticity and neuronal functioning. Rapid-eye-movement (REM) sleep has been shown to promote calcium-dependent plasticity in principal neurons of the cerebral cortex, both during memory consolidation in adults and during post-natal development. This article reviews the plasticity mechanisms triggered by REM sleep, with a focus on the emerging role of kinases and immediate-early genes for the progressive corticalization of hippocampus-dependent memories. The body of evidence suggests that memory corticalization triggered by REM sleep is a systemic phenomenon with cellular and molecular causes.

On Energy Efficiency and the Brain's Resistance to Change: The Neurological Evolution of Dogmatism and Close-Mindedness

The brain has been described as very resistant to change. However, why it does this has not been fully explained. In this paper, I propose that resilience to the disruption of consolidated memory is at the heart of the brain's resistance to change, and this resilience is a consequence of its energy efficiency evolutionary adaptations. I discussed the implications of this energy efficiency adaptation on dogmatism, close-mindedness, and artificial intelligence.

mTORC1 controls long-term memory retrieval

Understanding how stored information emerges is a main question in the neurobiology of memory that is now increasingly gaining attention. However, molecular events underlying this memory stage, including involvement of protein synthesis, are not well defined. Mammalian target of rapamycin complex 1 (mTORC1), a central regulator of protein synthesis, has been implicated in synaptic plasticity and is required for memory formation. Using inhibitory avoidance (IA), we evaluated the role of mTORC1 in memory retrieval. Infusion of a selective mTORC1 inhibitor, rapamycin, into the dorsal hippocampus 15 or 40 min but not 3 h before testing at 24 h reversibly disrupted memory expression even in animals that had already expressed IA memory. Emetine, a general protein synthesis inhibitor, provoked a similar impairment. mTORC1 inhibition did not interfere with short-term memory retrieval. When infused before test at 7 or 14 but not at 28 days after training, rapamycin impaired memory expression. mTORC1 blockade in retrosplenial cortex, another structure required for IA memory, also impaired memory retention. In addition, pretest intrahippocampal rapamycin infusion impaired object location memory retrieval. Our results support the idea that ongoing protein synthesis mediated by activation of mTORC1 pathway is necessary for long but not for short term memory.

Rapid, experience-dependent translation of neurogranin enables memory encoding

De novo protein synthesis is critical for memory formation. We found that protein synthesis during acquisition is transiently required for contextual memory formation. We identified one candidate gene, Nrgn (encoding protein neurogranin, Ng) with enhanced translation upon novel-context exposure, and found that experience-dependent translation of Ng in the hippocampus is required for contextual memory formation. Furthermore, fragile-X mental retardation protein interacts with the 3′UTR of the Nrgn mRNA, which is required for activity-dependent translation of Ng in the synaptic compartment and contextual memory formation. Together, these results indicate that experience-dependent and acute translation of Ng in the hippocampus during memory acquisition enables durable context memory encoding.

Experience induces de novo protein synthesis in the brain and protein synthesis is required for long-term memory. It is important to define the critical temporal window of protein synthesis and identify newly synthesized proteins required for memory formation. Using a behavioral paradigm that temporally separates the contextual exposure from the association with fear, we found that protein synthesis during the transient window of context exposure is required for contextual memory formation. Among an array of putative activity-dependent translational neuronal targets tested, we identified one candidate, a schizophrenia-associated candidate mRNA, neurogranin (Ng, encoded by the Nrgn gene) responding to novel-context exposure. The Ng mRNA was recruited to the actively translating mRNA pool upon novel-context exposure, and its protein levels were rapidly increased in the hippocampus. By specifically blocking activity-dependent translation of Ng using virus-mediated molecular perturbation, we show that experience-dependent translation of Ng in the hippocampus is required for contextual memory formation. We further interrogated the molecular mechanism underlying the experience-dependent translation of Ng, and found that fragile-X mental retardation protein (FMRP) interacts with the 3′UTR of the Nrgn mRNA and is required for activity-dependent translation of Ng in the synaptic compartment and contextual memory formation. Our results reveal that FMRP-mediated, experience-dependent, rapid enhancement of Ng translation in the hippocampus during the memory acquisition enables durable context memory encoding.